Antenna for rfid

ABSTRACT

An antenna for RFID which can be mounted directly on the surface of a management object without using a spacer whatever material the surface of the object might be made of. This antenna for RFID ( 14 ) is electrically connected with an IC chip or capacitor and mounted on an article ( 11 ). The antenna for RFID ( 14 ) comprises a conductive member ( 14   a ) formed into a flat plate and mounted on the article ( 11 ) at its back and a coil body ( 14   b ) fixed directly or with a predetermined spacing on a conductive member, spirally wound and having such a number of turns or a diameter that it has a predetermined characteristics value in the wound state.

TECHNICAL FIELD

[0001] The present invention relates to an antenna used for a tag usingthe RFID (Radio Frequency Identification) technique. More particularly,the present invention relates to an antenna for RFID which has a spiralcoil and will be set to an article.

BACKGROUND ART

[0002] A tag is known which has an antenna and an IC chip, storinginformation about an article to be managed, electrically connected tothe antenna, as a tag using the RFID technique. By transmitting radiowaves at a predetermined frequency to the antenna from the transceivingantenna of an interrogator, the tag is activated, data is read from theIC chip in accordance with a read command through data communication byradio waves, and data is written in the IC chip in accordance with awrite command. Moreover, a 1-bit RFID used for simplest EAS (ElectronicArticle Surveillance) is constituted by a capacitor and an antenna,which resonates by receiving a predetermined frequency and detectspresence or absence of an article under surveillance.

[0003] In order to minimize the thickness of the tag, the aboveconventional antenna for RFID used for the tag uses an antenna formed byforming a conductive wire whose surface is covered with an insulatinglayer into a nearly squared spiral and attaching it to a base plate oran antenna which is spiral by removing unnecessary portions of aconductive layer formed by an aluminum foil or copper foil throughetching or punching. In the case of a tag having the above antenna, whenan article to be managed is made of a metal, the tag is affixed to thearticle by using a machine screw or the like while inserting a spacerhaving a thickness of 5 to 10 mm and electrical insulating propertiesbetween the tag and article in order to avoid the influences of themetallic article.

[0004] However, in the case of the above conventional tag, because thespacer has a relatively large thickness, the interval between themetallic article and tag becomes relatively large, even if the thicknessof the antenna can be decreased, and a problem thereby occurs in thatthe tag greatly protrudes from the article to be managed. Therefore, thetag may contact a neighboring object while the article is beingtransported.

[0005] It is an object of the present invention to provide an antennafor RFID which can be directly affixed to an article to be managed,without using a spacer, no matter the kind of material making up thesurface of the article.

DISCLOSURE OF THE INVENTION

[0006] The present invention relates to improvement of an antenna forRFID to be affixed to an article electrically connected to an IC chip.The antenna for RFID has a conductive member which is formed into a flatplate and whose back will be affixed to an article and a coil body whichis made spiral so as to face the conductive member and at least one ofthe number of turns and the spiral diameter thereof is adjusted so as toobtain a predetermined characteristic value when the coil body is madespiral.

[0007] In the case of an antenna for RFID of the present invention, thebody of a coil is adjusted so that a predetermined characteristic valuecan be obtained when the body of the coil is wound on the surface of aconductive member. Therefore, it is possible to reliably activate a tagby transmitting radio waves at a predetermined frequency to the antennafor RFID from the transceiving (transmitting and/or receiving) antennaof an interrogator. Moreover, in the case of this antenna, because thebody of the coil is already wound on the front of the conductive memberand the predetermined characteristic value is obtained, the antenna isnot influenced by the metal in an article to be managed even if theantenna is directly affixed to the article, and the characteristic valueof the coil body is therefore not changed extremely. Thus, a spacer,which has required to affix an antenna to a metallic article, isunnecessary, and it is possible to avoid the tag greatly protruding froman article to be managed.

[0008] In this case, it is preferable that a conductive member use asheet, plate, foil, or spiral conductor whose both ends are connected toeach other and that the electrical resistance betweenlongitudinal-directional both ends of the conductive member having awidth of 1 cm and a length of 1 cm be 5 Ω or less. In this case, it ispossible for the electrical resistance to be equal to a value of 5 Ω orless obtained by changing the material and thickness of the conductivemember and dividing the specific electric resistance (electricresistivity) by the thickness, and it is possible to optionally selectthe material and thickness of the conductive member.

[0009] Moreover, it is possible for the conductive member to use aconductive paint film formed by applying a conductive ink to the back ofa sheet, plate, or foil made of a nonconductive material such aspolyethylene or polyethylene terephthalate and a conductive metalplating or deposited layer made of Cu or Al layered on the back of thesheet, plate, or foil of the nonconductive material. In this case, it ispreferable to set the gap between the conductive member and coil body to0.01 to 5 mm by using a sheet, plate, or foil with a thickness of 0.01to 5 mm.

[0010] Moreover, it is possible to form a hole at a portion of theconductive member surrounded by the coil body and to set a soft magneticmember between the conductive member and coil body. When forming a holeon the conductive member corresponding the central portion of the coilbody, even if an eddy current is generated on the conductive member dueto radio waves passing through the coil body, the eddy current isgenerated in a small range close to the coil body, and thus, it ispossible to prevent the Q value of the coil body from decreasing.Furthermore, when an object to be set is made of an insulating materialor a material allowing electromagnetic waves to pass through, such as ametallic deposited film, it is also possible to use electromagneticwaves passing through the hole of the conductive member, and thereby,the sensitivity of the tag is improved. Alternatively, when setting asoft magnetic member between the conductive member and coil body,because magnetic fluxes of the electromagnetic waves flow through thesoft magnetic member at a high density, a desired sensitivity can beobtained even if the interval between the conductive member serving asan electric conductor and the coil body serving as a coil is small.Moreover, it is allowed that the number of turns and/or spiral diameterof a coil for obtaining a predetermined inductance is small and the Qvalue is also improved.

[0011] Furthermore, an antenna for RFID in a second aspect of thepresent invention has a soft magnetic member which is formed into a flatplate and whose back will be set to an article and a coil body whosenumber of turns and/or spiral diameter is adjusted so as to obtain apredetermined characteristic value while it is spiral and affixed to thesurface of the soft magnetic member.

[0012] In the case of the antenna for RFID of the second aspect of thepresent invention, by transmitting radio waves while setting the antennaon an article formed of metal, a soft magnetic member prevents radiowaves from passing through a metallic portion and thereby, no eddycurrent is generated on the metallic portion. As a result, a tag isactivated, a spacer has been previously required is unnecessary, and itis possible to prevent the antenna from contacting with a neighboringobject when the article is transported.

[0013] In the case of a soft magnetic member, it is preferable that theproduct of the magnetic permeability and the thickness shown inmillimeters of the soft magnetic member be 0.5 or more. Moreover, it ispreferable that a soft magnetic member be formed by a soft magneticmaterial of any one of a rapidly solidifying material, casting material,rolling material, forging material, and sintering material of anamorphous alloy, permalloy, magnetic steel, sillicon steel, sendustalloy, Fe—Al alloy, or soft-magnetic ferrite. Furthermore, it ispossible for the soft magnetic member to be a composite material of fineparticles or flakes of a metal or ferrite and plastic or rubber, or apaint film containing fine particles of flakes of a metal or ferrite.Furthermore, it is possible for a soft magnetic member to be an adhesionsheet obtained by adhering a plurality of flakes made of a metal orsoft-magnetic ferrite to the surface of a base-material sheet made ofplastic so that the flakes closely contact with each other. Furthermore,it is possible to use a laminated sheet obtained by arranging aplurality of flakes made of a metal or soft-magnetic ferrite on thesurface of a base-material sheet made of plastic so that the flakesclosely contact each other, covering the flakes with a cover sheet madeof plastic, and adhering the base-material sheet and the cover sheettogether.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a top view of a tag including an antenna for RFID of afirst embodiment of the present invention;

[0015]FIG. 2 is a sectional view taken along the line A-A of FIG. 1;

[0016]FIG. 3 is a top view of a tag including an antenna on whoseconductive member a hole is formed corresponding to FIG. 1;

[0017]FIG. 4 is a sectional view of the antenna in FIG. 3 correspondingto FIG. 2;

[0018]FIG. 5 is a sectional view, corresponding to FIG. 2, showing anantenna of a second embodiment in which a soft magnetic member is set;

[0019]FIG. 6 is a sectional view, corresponding to FIG. 2, showing anantenna of a third embodiment in which a soft magnetic member is affixedto an article;

[0020]FIG. 7 is a top view, corresponding to FIG. 1, showing a case inwhich the body of a coil is quadrangular; and

[0021]FIG. 8 is an illustration showing a state of confirming operationsof a tag of an embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] Now, the first embodiment of the present invention is describedbelow with reference to the accompanying drawings.

[0023] As shown in FIGS. 1 and 2, an RFID tag 12 is affixed to thesurface of an article 11 and the tag 12 has an IC chip 13 storingspecific information which differs for every article 11 and an antennafor RFID 14 electrically connected to the IC chip 13. In the case of thearticle 11 of this embodiment, to a portion of which a tag is affixed isformed by a metallic material. The antenna 14 of the present inventionhas a conductive member 14 a formed into a flat plate by a conductivematerial is mounted on the article 11 at its back and a coil body 14 bformed spirally at the front side of the conductive member 14 a.

[0024] The conductive member 14 a uses a sheet, plate, or foil made of aconductive material such as copper or aluminum and it is possible to usea spiral electric conductor whose both ends are connected to each other.Moreover, as long as the conductive member 14 a has conductingproperties, it is possible for the member 14 a to be a conductive paintfilm obtained by applying a conductive ink to the back of anonconductive sheet, plate, or foil 16 made of polyethylene orpolyethylene terephthalate and drying it as shown in the enlarged viewin FIG. 2. It is preferable that a conductive paint contain particles orflakes of silver or graphite. Furthermore, it is possible for theconductive member 14 a to be a conductive metal plating layer ordeposited film laminated on the back of the nonconductive sheet, plate,or foil 16. When using the conductive member 14 a made of a paint film,metal plating, or deposited layer, it is preferable that thenonconductive sheet, plate or foil 16 have a thickness of 0.01 to 5 mm.By setting the thickness of the sheet, plate, or foil 16 to 0.01 to 5mm, the interval between the conductive member 14 a and coil body 14 bincreases, and it is possible to improve the Q value of the coil body 14b and the performance as an antenna. Moreover, it is preferable that theelectric resistance for a width of 1 cm and length of 1 cm of theconductive member 14 a be 5 Ω or less.

[0025] The spiral coil body 14 b uses a conventional coil body havingbeen used so far. That is, the coil body 14 b is formed by winding acoated copper wire or uses a coil body formed by removing unnecessaryportions of a conductive layer such as an aluminum foil or copper foillaminated on an insulating plastic sheet through etching or thin-platepunching and forming a spiral. When the coil body 14 b is formed by analuminum foil or copper foil, it is necessary to fix the coil body 14 bto the front of the conductive member 14 a through an insulating film(not shown) in order to prevent the coil body 14 b from beingelectrically connected with the conductive member 14 a. Moreover, whenthe conductive member 14 a is formed by a nonconductive sheet, plate, orfoil and a paint film, metal plating, or deposited layer formed on theback of the sheet, plate, or foil, it is allowed to directly laminate analuminum foil or copper foil on the surface of the nonconductive sheet,plate, or foil, remove unnecessary portions of the aluminum or copperfoil through etching, and directly form the spiral coil body 14 b on thesurface of the aluminum or copper foil. The coil body 14 b is formed byadjusting the number of turns and/or spiral diameter of the coil body 14b so that a predetermined characteristic value can be secured while thecoil body 14 b is wound on the front of the conductive member 14 a. TheIC chip 13 of this embodiment is directly adhered onto the conductivemember 14 a while it is connected to both ends of the coil body 14 b.

[0026] In the case of the antenna for RFID 14 constituted as describedabove, by fixing the coil body 14 b to the conductive member 14 a, thecharacteristic value generally changes. However, because the coil body14 b is adjusted so that a predetermined characteristic value can besecured while it is wound on the front of the conductive member 14 a, itis possible to reliably activate the tag 12 by transmitting radio wavesat a predetermined frequency to the antenna 14 from the transceivingantenna of an interrogator (not shown). Moreover, in the case of theantenna 14, the coil body 14 b for actually receiving radio waves at apredetermined frequency is already wound on the front of the conductivemember 14 a and a predetermined characteristic value is secured.Therefore, even if the antenna 14 is directly affixed to a metallicarticle, the characteristic value of the coil body 14 b is not extremelychanged. Therefore, a spacer, which has been necessary to affix theantenna 14 to a metallic article, is unnecessary, and it is possible toprevent the tag 12 from greatly protruding from an article and toreliably activate the tag 12 without being influenced by the metallicarticle.

[0027] As shown in FIGS. 3 and 4, by forming a hole 14 c at a portion ofthe conductive member 14 a surrounded by the coil body 14 b, change ofinductances and deterioration of the Q value of the coil due to thesurface of an article are maintained within their tolerances when thesurface is formed of a conducive material or magnetic material becausethe periphery of the coil is shielded by a conductive material.Moreover, when an article is made of a material allowing radio waves topass, such as an insulating material or metallic deposited film, thesensitivity of a tag is improved because it is possible to use radiowaves which pass through a hole but which cannot be used when there isno hole.

[0028]FIG. 5 shows a second embodiment of the present invention. In FIG.5, the same reference numerals as in FIGS. 1 and 2 are used to indicatethe same components.

[0029] This embodiment uses an antenna for RFID 24 in which a flat softmagnetic member 26 is set between the conductive member 14 a and coilbody 14 b of the antenna of the above first embodiment. In this case,because details of the conductive member 14 a and coil body 14 b are thesame as those of the first embodiment, repetitive description isomitted.

[0030] It is preferable that the soft magnetic member 26 is formed byany one of soft magnetic materials such as a rapidly solidifyingmaterial, casting material, rolling material, forging material, andsintering material of an amorphous alloy, permalloy, magnetic steel,sillicon steel, sendust alloy, Fe—Al alloy, or soft-magnetic ferrite andthe product of the magnetic permeability and the thickness shown inmillimeters of the soft magnetic member 26 is 0.5 or more. Moreover, aslong as the soft magnetic member 26 has magnetism, it is possible forthe member 26 to be a composite material of fine particles or flakes ofa metal or ferrite and plastic or rubber, or a paint film of a paintcontaining fine particles or flakes of a metal or ferrite. In this case,as the plastic of the compound material, it is possible to use athermoplastic resin which is superior in workability or a thermosettingresin which is superior in heat resistance. Moreover, metallic fineparticles include carbonyl iron powder, atomized powder such asiron-permalloy, and reduced iron powder. Alternatively, it is possiblefor the metallic flakes to be flakes obtained by fining the above powderby a ball mill and then mechanically flattening the powder or flakesobtained by making molten-metal particles of an iron-based orcobalt-based amorphous alloy collide with water-cooled copper.

[0031] Furthermore, when using a plurality of flakes made of a metal orsoft-magnetic ferrite, it is allowed to constitute the soft magneticmember 26 by an adhesive sheet obtained by adhering flakes to thesurface of a base-material sheet made of plastic so that the flakesclosely contact with each other or constitute the soft magnetic member26 by a laminated sheet obtained by arranging a plurality of flakes madeof a metal or soft magnetic ferrite on the surface of a base-materialsheet made of plastic so that the flakes closely contact with eachother, covering the flakes with a cover sheet made of plastic, andadhering the base-material sheet and the cover sheet together.

[0032] Furthermore, to use a compound material as the soft magneticmember 26, it is possible to form the soft magnetic material 26 byinjection-molding or compression-molding the member 26. The softmagnetic member 26 thus formed does not easily crack even if itsthickness decreases because it is tough compared to a soft magneticmember formed by weak ferrite. Moreover, because fine particles offlakes of a metal or ferrite are distributed into plastic or rubber andare insulated from each other, the soft magnetic member 26 is notconductive as a whole and eddy currents do not occur even when receivinghigh-frequency radio waves.

[0033] When the soft magnetic member 26 is made of a composite material,it is preferable to form fine particles or flakes so that thethicknesses thereof are 74 μm or less, in order to prevent eddy currentsfrom occuring in the fine particles or flakes of a metal or ferrite.Moreover, as a plastic, it is preferable to use any one of acrylic,polyester, polyvinyl-chloride, polyethylene, polystyrene, and epoxyresins. In this case, the thickness of the soft magnetic member 26 isnot restricted as long as the electromagnetic shielding effect isexhibited. However, it is preferable that the thickness range between 20and 3,000 μm for practical use.

[0034] In the case of the antenna for RFID 24 thus constituted, becausemagnetic fluxes of electromagnetic waves flow through the soft magneticmember 26 at a high density, a desired sensitive is obtained even if theinterval between the conductive member 14 a and coil body 14 b is small,it is possible for the number of turns and/or spiral diameter of a coilfor obtaining a predetermined conductance to be small, and the Q valueis improved. As a result, a spacer, which has been required to affix theantenna 24 to a metallic article, is unnecessary, the tag 12 isprevented from protruding from an article to be managed, and it ispossible to securely activate the tag 12 without being influenced by themetallic article.

[0035]FIG. 6 shows the third embodiment of the present invention. InFIG. 6, reference numbers the same as those in FIGS. 1 to 5 indicate thecorresponding parts.

[0036] An antenna for RFID 34 of this embodiment has a soft magneticmember 26 formed into a flat plate whose back is set to an article and acoil body 14 b whose number of turns and/or spiral diameter is adjustedso as to obtain a predetermined characteristic value in a spiral shapewound and fixed on the surface of the soft magnetic member 26. Detailsof the coil body 14 b are the same as the case of the first embodimentand the soft magnetic member 26 is the same as the case of the secondembodiment. The antenna for RFID 14 thus constituted is effective when apredetermined characteristic value of the coil body 14 b can be securedonly through electromagnetic shielding of the soft magnetic member 26.

[0037] In the case of the antenna for RFID 34, the electromagneticshielding of the soft magnetic member 26 ensures that a predeterminedcharacteristic value can be reliably obtained even when affixing theantenna 34 to the surface of a metallic article, and it is possible toreliably activate the tag 12 by transmitting radio waves at apredetermined frequency to the antenna 34 from a transceiving antenna ofan interrogator (not shown). Therefore, a spacer, which has beenrequired to affix the antenna 34 to a metallic article, is unnecessary,the tag 12 can be prevented from protruding from an article to bemanaged, and it is possible to reliably activate the tag 12 withoutbeing influenced by the metallic article.

[0038] In the case of the first to third embodiments, the coil body 14 bis formed into a nearly circular spiral shape. However, it is alsopossible to form the coil body 14 b into a nearly elliptical spiralshape, a nearly square spiral shape shown in FIG. 7, or another spiralshape.

EXPERIMENTAL EXAMPLES

[0039] Next, Experimental Examples of the present invention aredescribed below in detail together with Comparative Examples.

Experimental Example 1

[0040] As shown in FIGS. 1 and 2, a coil body with an outer diameter of50 mm and an inner diameter of 49 mm was made by winding a coated copperwire having a diameter of 0.2 mm four to five times. Moreover, amild-steel plate having a size of 100×100 mm and a thickness of 0.16 mmwas prepared as an article and a non-metallic acrylic plate having thesame shape and size as the mild-steel plate was also prepared forcomparison. As a conductive member, an aluminum plate having a size of50×50 mm and a thickness of 0.2 mm was affixed to the surfaces of themild-steel plate and acrylic plate, respectively. Then, the L1 value andthe Q1 value of the coil body were measured by directly affixing thecoil body at the surface of the aluminum plate which is affixed on themild-steel plate, or by maintaining a predetermined interval to thesurface of the aluminum plate. Thereafter, the values of L2 and Q2 weremeasured by directly affixing the coil body at the surface of thealuminum plate which is affixed on the acrylic plate, or by maintaininga predetermined interval to the surface of the aluminum plate.

Experimental Example 2

[0041] A soft magnetic member was set between the coil body and aluminumplate of the Experimental Example 1. The soft magnetic member used amember having an outer diameter of 60 mm and a thickness of 0.34 mmwhich was obtained by injection-molding a composite material made of 72%of carbonyl iron and polyethylene and then compressing the mouldedcomposite material. The coil body of the Experimental Example 1 wasbrought into close contact with the surface of the soft magnetic member26 to measure the L1 value and the Q1 value of the coil body by directlybringing the back of the soft magnetic member 26 into contact with thesurface of an aluminum plate set on the mild-steel plate of theExperimental Example 1 or setting the soft magnetic member 26 bymaintaining a predetermined interval. Then, the L2 value and the Q2value of the coil body were measured by directly bringing the back ofthe soft magnetic member 26 into contact with the surface of thealuminum plate set on the acrylic plate of the Experimental Example 1 ordisposing the soft magnetic member 26 by maintaining a predeterminedinterval therebetween. Then, L1/L2 was obtained.

Experimental Example 3

[0042] L1 and Q1 values and L2 and Q2 values of a coil body weremeasured in a manner similar to that in Experimental Example 1, exceptthat an aluminum foil having a thickness of 10 μm was used instead ofthe aluminum plate of the Experimental Example 1, and L1/L2 wasdetermined.

Experimental Example 4

[0043] L1 and Q1 values and L2 and Q2 values of a coil body weremeasured in a manner similar to that in Experimental Example 1, exceptthat an aluminum foil having a thickness of 10 μm was used instead ofthe aluminum plate of the Experimental Example 2, and L1/L2 wasdetermined.

Comparative Example 1

[0044] L1 and Q1 values of a coil body were measured by setting the coilbody to the surface of a mild-steel plate serving as the article of theExperimental Example 1 directly or by maintaining a predeterminedinterval therebetween. Moreover, L2 and Q2 values of a coil body weremeasured by disposing the coil body at the surface of an acrylic platedirectly or by maintaining a predetermined interval therebetween. Then,L1/L2 was determined. Table 1 shows the above measurement results. TABLE1 Mild-steel Acrylic plate plate L2 L1 Article (μH) Q2 (μH) Q1 L1/L2Experimental Gap 0 0.667 15.5 0.677 15.2 99 Example 1 between 0.6071.163 25.1 1.204 31.7 97 aluminum 1.214 1.482 45.5 1.511 34.4 98 plateand 1.821 1.694 52.7 1.702 49.4 100 coil body 2.428 1.892 57.2 1.86254.9 102 Experimental Gap 0 1.804 40.0 1.775 38.5 102 Example 2 between0.607 2.019 43.9 2.021 42.2 100 aluminum 1.214 2.187 46.9 2.182 44.6 100plate and 1.821 2.338 47.8 2.330 47.0 100 soft 2.428 magnetic memberExperimental Gap 0 0.541 8.5 0.567 8.8 95 Example 3 between 0.607 1.18727.7 1.172 27.5 101 aluminum 1.214 1.547 39.1 1.499 37.3 103 foil and1.821 1.763 45.3 1.749 45.0 101 coil body 2.428 1.905 48.5 1.879 47.8101 Experimental Gap 0 1.855 33.1 1.969 22.0 94 Example 4 between 0.6072.129 36.4 2.146 36.6 99 aluminum- 1.214 2.281 38.2 2.329 37.9 98 foiland 1.821 2.456 39.9 2.507 39.1 98 soft 2.428 2.586 39.3 2.549 39.5 101magnetic member Comparative Gap 0 2.968 77.2 1.213 6.8 245 Example 1between 0.607 2.968 77.2 1.359 9.3 218 metallic 1.214 2.968 77.2 1.61214.5 184 plate and 1.821 2.968 77.2 1.801 19.3 165 coil body 2.428 2.96877.2 1.963 24.1 151

[0045] The measurement frequency used 13.56 MHz.

[0046] As shown in Table 1, in the case of the Comparative Example 1, itwas found that the value of L1/L2 was large, the change rate is largewhen directly affixing a coil body on a metal, and a tag cannot beactivated when the coil body is actually used for the tag. Moreover, thefact is clarified that as the interval between the coil body and themetal increases, the change rate decreases and a tag cannot be activatedunless the conventional antenna constituted by only the coil body isaffixed to the metal surface through a spacer having a predeterminedthickness.

[0047] In the case of the Experimental Examples 1 and 3 in which analuminum plate or aluminum foil is set between a metallic plate and acoil body, it was found that the value of L1/L2 showing a change of Lextremely decreases. Therefore, even if directly affixing an antenna forRFID to which a coil body whose number of turns and/or spiral diameteris adjusted so as to obtain a predetermined characteristic value whilethe coil body is wound on the surface of a conductive member is affixedto a metallic article, it can be expected that functions of the antennacan be exhibited and it was found that the present invention iseffected.

[0048] Moreover, in the case of the Experimental Examples 2 and 4 inwhich a soft magnetic member is further disposed between an aluminumplate or aluminum foil and a coil body, it was found that the value ofL1/L2 was extremely decreased compared to the case of the ComparativeExample 1 and the Q value was improved compared to the case of theExperimental Examples 1 and 3. Ω values are increased, loss due to eddycurrents are decreased, and characteristics of an antenna for RFID areimproved. Therefore, in the case of the present invention in which asoft magnetic member is disposed between a conductive member and a coilbody, it was found that functions of an antenna can be sufficientlyimproved.

[0049] Now, an Experimental Example showing whether a tag using anantenna of the present invention actually operates is described below indetail together with a Comparative Example.

Experimental Example 5

[0050] As shown in FIGS. 1 and 2, a coil body having an outer diameterof 50 mm and an inner diameter of 49 mm was made by winding a coatedcopper wire having a diameter of 0.2 mm four to five times. An aluminumfoil having a thickness of 10 μm and a size of 60×60 mm was prepared asa conductive member. An RFID tag was obtained by directly affixing thecoil body to the surface of the aluminum foil and electricallyconnecting an IC chip to the coil body. The tag was used as theExperimental Example 5

Experimental Example 6

[0051] A coil body the same as that used for the Experimental Example 5was made in accordance with the same procedures as in the case of theExperimental Example 5. Moreover, an aluminum foil having the same shapeand same size as the case of the Experimental Example 5 and an acrylicplate having the same outer-diameter as the aluminum foil and athickness of 0.607 mm were prepared. An RFID tag was obtained byaffixing the coil body to the surface of the aluminum foil through theacrylic plate and electrically connecting an IC chip to the coil body.Thus, the tag to which the coil body was affixed by maintaining aninterval of 0.607 mm from the aluminum foil was used as the ExperimentalExample 6.

Experimental Example 7

[0052] A coil body the same as that used for the Experimental Example 5was made in accordance with the same procedures as in the case of theExperimental Example 5. Moreover, an aluminum foil having the same shapeand size as the case of the Experimental Example 5 was prepared to forma circular hole having a diameter of 40 mm at the center of the aluminumfoil. A coil body was affixed to the surface of the aluminum foil so asto surround the circular hole and an RFID tag was obtained byelectrically connecting an IC chip to the coil body. The tag in whichthe coil body was affixed to the aluminum foil on which the circularhole is formed was used as the Experimental Example 7.

Experimental Example 8

[0053] A coil body the same as that used for the Experimental Example 5was made in accordance with the same procedures as in the case of theExperimental Example 5. Moreover, an aluminum foil serving as aconductive member having the same shape and size as the case of theExperimental Example 5 and a composite material including carbonyl ironhaving a thickness of 0.34 mm and a size of 60×60 mm serving as a softmagnetic member were prepared. A coil body was affixed to the surface ofthe aluminum foil through the compound material and an RFID tag wasobtained by electrically connecting an IC chip to the coil body. Thus,the tag, in which a flat soft magnetic member was disposed between theconductive member and coil body, was used as the Experimental Example 8.

Experimental Example 9

[0054] A coil body the same as that used for the Experimental Example 5was made in accordance with the same procedures as in the case of theExperimental Example 5. Moreover, an aluminum foil serving as aconductive member having the same shape and size as the case of theExperimental Example 5 was prepared and a paint containing flakymagnetic powder was applied to the aluminum foil and was dried to form apaint film having a thickness of 0.2 mm serving as a soft magneticmember on the surface of the aluminum foil. The coil body was affixed tothe surface of the paint film and an RFID tag was obtained byelectrically connecting an IC chip to the coil body. The tag in which apaint film serving as a soft magnetic member was formed between theconductive member and coil body was used as the Experimental Example 9.

Experimental Example 10

[0055] A coil body the same as that used for the Experimental Example 5was made in accordance with the same procedures as in the case of theExperimental Example 5. Moreover, a composite material containingcarbonyl iron having a thickness of 0.34 mm and a size of 60×60 mm wasprepared as a soft magnetic member. A paint containing silver powder wasapplied to the back of the composite material and was dried to form apaint film serving as a conductive member having a thickness of 0.15 mmon the back of the composite material. Moreover, the coil body wasaffixed to the surface of the composite material and an RFID tag wasobtained by electrically connecting an IC chip to the coil body. The tagin which the composite material serving as a soft magnetic member wasdisposed between the paint film and coil body was used as theExperimental Example 10.

Experimental Example 11

[0056] A coil body the same as that used for the Experimental Example 5was made in accordance with the same procedures as in the case of theExperimental Example 5. A paint film having a thickness of 0.15 mmserving as a conductive member was formed on the back of a compositematerial in accordance with the same procedure as the case of theExperimental Example 10. The paint film is peeled off the compoundmaterial. A coil body was fixed to the surface of the paint film servingas a conductive member and an RFID tag was obtained by electricallyconnecting an IC chip to the coil body. The tag having the paint filmserving as a conductive member was used as the Experimental Example 11.

Experimental Example 12

[0057] A coil body the same as that used for the Experimental Example 5was made in accordance with the same procedures as in the case of theExperimental Example 5. A discoid ferrite plate having a thickness of 1mm and a diameter of 60 mm was prepared as a conductive member. The coilbody was directly affixed to the surface of the ferrite plate and anRFID tag was obtained by electrically connecting an IC chip to the coilbody. The tag was used as the Experimental Example 12.

Comparative Example 2

[0058] A coil body the same as that used for the Experimental Example 5was made in accordance with the same procedures as in the case of theExperimental Example 5. An RFID tag was obtained by electricallyconnecting an IC chip to the coil body. The tag constituted by the coilbody and IC chip was used as the Comparative Example 2.

Comparative Example 3

[0059] A coil body obtained by winding a coated copper wire having adiameter of 0.2 mm on a composite material containing carbonyl ironhaving a thickness of 1 mm and a size of 40×40 mm ten times was made. AnRFID tag was obtained by electrically connecting an IC chip to the coilbody. The tag constituted by the bobbin-type coil body and IC chip wasused as the Comparative Example 3.

Comparative Test and Evaluation

[0060] Thicknesses of the tags of the Experimental Examples 5 to 11 andComparative Examples 2 and 3 were measured and L3 and Q3 values of coilbodies were measured by arranging the tags on an acrylic plate.Moreover, it was determined whether the taf was normally operated bybringing the transceiving antenna 21 a of the interrogator 21 shown inFIG. 8 close to the tag up to 30 mm.

[0061] Thereafter, as shown in FIG. 8, these tags 12 were arranged on aniron plate having a thickness of 1 mm serving as the article 11 tomeasure L4 and Q4 values of the coil bodies. Then, it was confirmedwhether the tag was normally operated by bringing the transceivingantenna 21 a of the interrogator 21 near to the tag 12 up to 30 mm.

[0062] The measured values of L3, L4 and Q3, Q4, and the effect of theoperation (operable or inopereble) is listed in Table 2. TABLE 2 Upperstage L3 (μH) Configuration Thickness Lower stage Upper stage Q3 Effectof of tag (mm) L4 (μH) Lower stage Q4 operation Experimental Aluminumfoil 0.45 0.541 8.5 Operable Example 5 0.567 8.8 Operable ExperimentalAluminum foil 1.06 1.187 27.7 Operable Example 6 and gap 1.172 27.5Operable Experimental Circular-hole- 0.45 1.127 27.0 Operable Example 7provided 1.067 13.8 Operable aluminum foil Experimental Compound 0.851.855 33.1 Operable Example 8 material and 1.969 22.0 Operable aluminumfoil Experimental Magnetic paint 0.7 1.754 36.7 Operable Example 9 filmand 1.844 36.8 Operable aluminum foil Experimental Compound 0.8 1.79036.8 Operable Example 10 material and 1.8207 36.9 Operable conductivepaint film Experimental Conductive 0.5 0.808 11.4 Operable Example 11paint film 0.781 12.8 Operable Experimental Ferrite plate 1.5 4.611 60.3Operable Example 12 4.261 38.9 Operable Comparative Coil only 0.4 2.96877.2 Operable Example 2 1.213 6.8 Inoperable Comparative Bobbin-type 2.04.356 88.4 Operable Example 3 coil 4.449 60.4 Inoperable

[0063] The measurement frequency used 13.56 MHz.

[0064] From Table 2, it was found that changes of L and Q when arrangingtags on a metal are large in the case of the Comparative Example 2 andthe tags when arranged on the metal do not operate normally. Moreover,in the case of the Comparative Example 3, changes of L and Q whenarranging tags on a metal are small compared to the case of theComparative Example 2, but the tags did not operate normally whenarranged on the metal. It is estimated that the above-described matteris caused by the fact that because the magnetic-core direction of theso-called bobbin-type coil coincides with the axial-core direction ofthe coil, radio waves cannot be transmitted from the transceivingantenna of an interrogator approached from the direction orthogonal tothe surface of a metal serving as an article.

[0065] In the case of the Experimental Examples 5 to 9 respectivelyhaving a conductive member formed by an aluminum foil and theExperimental Examples 10 and 11 respectively having a conductive memberformed by a conductive paint film, it was found that values of L and Qare relatively small, but their changes are small, and tags whenarranged on a metal operate normally. Moreover, it was found that valuesof L and Q are improved in the case of the Experimental Example 6provided with a gap and the Experimental Example 7 in which the circularhole is formed on the aluminum foil compared to the case of theExperimental Example 5 in which the coil body is directly fixed to thealuminum foil. Furthermore, in the case of the Experimental Examples 8to 10 in each of which the soft magnetic member is set between theconductive member and coil body, it was found that values of L and Q areimproved. Therefore, in the case of the present invention in which asoft magnetic member is set between a conductive member and a coil body,it was found that functions of an antenna are sufficiently improved.

[0066] Furthermore, in the case of the Experimental Example 12 in whichthe coil body is fixed to the surface of the ferrite plate serving as asoft magnetic member, it was found that values of L and Q are furtherimproved and the tag operate normally. Therefore, as long as apredetermined characteristic value of a coil body can be secured onlythrough electromagnetic shielding of a soft magnetic member, it wasfound that a predetermined characteristic value can be reliably obtainedeven when a tag is affixed to the surface of a metallic article and thetag can be operated normally.

Industrial Applicability

[0067] As described above, according to the present invention, thecharacteristic value of a coil body is not extremely changed even whendirectly affixing an antenna on a metallic article to be managed, and aspacer, which has been required to affix an antenna to a metallicarticle, is unnecessary, and a tag can be prevented from greatlyprotruding from an article to be managed.

[0068] Moreover, a coil body is electromagnetically shielded by a softmagnetic member set between a conductive member and the coil body, the Qvalue of the coil body is improved, adjustment of the number of turnsand/or spiral diameter of the coil body to be performed to obtain apredetermined characteristic value becomes relatively easy, and it ispossible to reliably obtain the predetermined characteristic value evenif affixing the antenna to the surface of a metallic article.

[0069] Furthermore, by affixing the back of a soft magnetic member to anarticle and affixing a spiral coil body to the surface of the softmagnetic member directly or by maintaining a predetermined intervaltherebetween, when transmitting radio waves while affixing an antenna toa metallic article, eddy currents are not generated at a metallicportion because the soft magnetic member prevents the radio waves frompassing through the metallic portion. As a result, a tag is activated, aspacer, which has been required, is unnecessary, even if an article ismade of metal, and it is possible to prevent the antenna from contactingwith a neighboring object while the article is transported.

1. An antenna for RFID electrically connected to an IC chip (13) orcapacitor and affixed to an article (11), comprising: a conductivemember (14 a) formed into a flat plate and mounted on the article (11)at its back; and a coil body (14 b) formed into a spiral and mounted atthe front side of the conductive member (14 a), wherein the number ofturns and diameter of said coil body are adjusted so that thecharacteristic of said coil body has a predetermined value.
 2. Theantenna for RFID according to claim 1, wherein the electrical resistancebetween both ends of the area having a width of 1 cm and a length of 1cm of said conductive member (14 a) in the longitudinal direction is 5 Ωor less.
 3. The antenna for RFID according to claim 1, wherein saidconductive member (14 a) is a sheet, plate, foil, or conductor in aspiral shape whose both ends are connected to each other.
 4. The antennafor RFID according to claim 1, wherein said conductive member (14 a) isa conductive paint film obtained by applying a conductive ink to theback of a nonconductive sheet (16), plate, or foil and drying it andsaid coil body (14 b) is wound on and fixed to the surface of said sheet(16), plate, or foil.
 5. The antenna for RFID according to claim 1,wherein said conductive member (14 a) is a conductive metal plating ordeposited film laminated on the back of a nonconductive sheet (16),plate, or foil and said coil body (14 b) is wound on and affixed to thesurface of said sheet (16), plate, or foil.
 6. The antenna for RFIDaccording to claim 1, wherein the gap between said conductive member (14a) and said coil body (14 b) ranges between 0.01 and 5 mm.
 7. Theantenna for RFID according to claim 1, wherein a hole (14 c) is formedat a portion of said conductive member (14 a) surrounded by said coilbody (14 b).
 8. The antenna for RFID according to claim 1, wherein asoft magnetic member (26) is disposed between said conductive member (14a) and said coil body (14 b).
 9. An antenna for RFID electricallyconnected to an IC chip (13) and affixed to an article (11), comprising:a soft magnetic member (26) formed into a flat plate and mounted on thearticle (11) at its back; and a coil body (14 b) mounted at the frontside of said soft magnetic member (26), wherein the number of turns anddiameter of said coil body (14 b) are set so that characteristics ofsaid coil body have predetermined values.
 10. The antenna for RFIDaccording to claim 8 or 9, wherein said soft magnetic member (26) has avalue of 0.5 or more in the product of the magnetic permeability and thethickness in millimeters of the member (26).
 11. The antenna for RFIDaccording to claim 8 or 9, wherein said soft magnetic member (26) isformed by any soft magnetic material such as a rapidly solidifiyingmaterial, casting material, rolling material, forging material, andsintering material of amorphous alloy, permalloy, magnetic steel,sillicon steel, sendust alloy, Fe—Al alloy, or soft-magnetic ferrite.12. The antenna for RFID according to claim 8 or 9, wherein said softmagnetic member (26) uses a composite material of fine particles orflakes of a metal or ferrite and plastic or rubber, or a film of a paintcontaining fine particles or flakes of a metal or ferrite.
 13. Theantenna for RFID according to claim 8 or 9, wherein said soft magneticmember (26) is an adhesive sheet obtained by adhering a plurality offlakes made of a metal or soft magnetic ferrite to the surface of abase-material sheet made of plastic so that the flakes closely contacteach other.
 14. The antenna for RFID according to claim 8 or 9, whereinsaid soft magnetic member (26) is a laminated sheet obtained byarranging a plurality of flakes made of a metal or soft magnetic ferriteon the surface of a base-material sheet made of plastic so that theflakes closely contact each other, covering the flakes with a coversheet made of plastic, and adhering said base-material sheet and saidcover sheet together.